The present invention relates to a screen printing apparatus for printing paste, such as cream solder or conductive paste, onto a substrate, and a method of the screen printing using the same apparatus.
In an electronic-parts-mounting process, a screen printing process is employed for printing paste, such as cream solder or conductive paste, onto a substrate. According to the screen printing process, a mask plate having pattern apertures corresponding to the places to be printed is placed over a substrate, and paste is printed onto the substrate through the pattern apertures by using a squeegee method.
According to a conventional squeegee method or apparatus, such as disclosed in U.S. Pat. No. 6,171,399, a so-called closed-type squeegee head which has a receptacle holding solder paste therein is known. According to the closed-type squeegee head, solder paste is not supplied to the mask plate separately from the squeegee head. Instead, solder paste held in the receptacle of the closed-type squeegee head is supplied to be filled in the pattern apertures of the mask plate when the squeegee head moves under the condition that a lower end of the squeegee head keeps a contact with the mask plate.
It is generally required for such a squeegee method or apparatus to fill solder paste in the pattern apertures of the mask plate and to wipe extra solder paste on the mask plate away. According to the conventional squeegee method and apparatus, a wiping characteristic deteriorates if the squeegee (wiper) of the squeegee head has a shape or an attachment angle such that a paste filling characteristic is well satisfied. On the other hand, the paste filling characteristic deteriorates if the squeegee has a shape or an attachment angle such that the wiping characteristic is well satisfied. Therefore, the conventional squeegee method and apparatus do not satisfy both of the paste filling characteristic and the wiping characteristic simultaneously.
Accordingly, it is an object of the present invention to provide a screen printing apparatus and method capable of satisfying both of the paste filling characteristic and the wiping characteristic simultaneously.
It is another object of the present invention to provide a screen printing apparatus and method capable of improving the paste filling characteristic of a closed-type squeegee head.
According to the first aspect of the present invention, a paste is filled into pattern apertures of a mask plate on a substrate by sliding a filling member provided at a lower end of a squeegee head. The filling member forms an acute angle with an upper surface of the mask plate in a space formed in the squeegee head for accommodating the paste. Any extra paste on the mask plate is wiped away (i.e., of the mask plate) by sliding a wiper provided at a lower end of the squeegee head across the mask plate. The wiper forms an obtuse angle with the upper surface of the mask plate in the space formed in the squeegee head for accommodating the paste.
Accordingly, both the paste filling characteristic and the wiping characteristic are satisfied simultaneously by the present invention.
According to another aspect of the present invention, a first pressure is applied to the paste accommodated in the squeegee head when the squeegee head is in a waiting position. Then, a second pressure smaller than the first pressure is applied to the paste accommodated in said squeegee head while the squeegee head is moving on the mask plate.
Accordingly, the paste filling characteristic of a closed-type squeegee head is improved.